Sizing Conductors, Part XX

Table 310.15(B)(16) (formerly Table 310.16) in the National Electrical Code (NEC) provides allowable ampacities for insulated conductors rated up to and including 2,000 volts (V). The ampacities in this table are based on no more than three current-carrying conductors in a raceway, cable or earth (directly buried). The ampacities are also based on an ambient temperature of 30°C.

When there are more than three current-carrying conductors and/or the ambient temperature is other than 30°C, the table ampacities must be adjusted and/or corrected. This is often referred to as derating. Factors, such as ambient temperature and the number of current-carrying conductors, must be considered when sizing conductors. Other factors, such as the temperature ratings of the terminations, continuous loads and overcurrent protective devices, must also be considered. Therefore, when sizing conductors, more is involved than just looking in Table 310.15(B)(16) and selecting a conductor based on the ampacity shown.

In accordance with 240.4, conductors (other than flexible cords, flexible cables and fixture wires) shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G). The rules in 240.4(A) through (G) are alternatives. One alternative provision is for overcurrent devices rated 800A or less. In accordance with 240.4(B), the next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the conditions in 240.4(B)(1) through (3) are met. This rule, in 240.4(B), is often referred to as the round-up rule. Where the overcurrent device is rated over 800A, the ampacity of the conductors it protects shall be equal to or greater than the rating of the overcurrent device defined in 240.6 [240.4(C)]. A new provision in Part VIII of Article 240 pertains to the protection of conductors and includes a rule for overcurrent devices rated over 800A. Because this new provision is in Part VIII, it only pertains to supervised industrial installations. This part covers overcurrent protection for those portions of supervised industrial installations operating at voltages of not more than 600V, nominal. Before covering this new provision, it is important to understand the term “supervised industrial installation.” A supervised industrial installation is only referenced in Article 240, and since it is only used in this one article, the definition is in Article 240 (240.2) instead of Article 100. For the purposes of Part VIII of Article 240, the industrial portions of a facility apply where all three of the following conditions are met: 1) conditions of maintenance and engineering supervision ensure that only qualified people monitor and service the system; 2) the premises wiring system has 2,500 kilovolt-amperes (kVA) or greater of load used in industrial process(es), manufacturing activities, or both, as calculated in accordance with Article 220; and 3) the premises has at least one service or feeder that is more than 150 volts-to-ground and more than 300V phase-to-phase. This definition excludes installations in buildings used by the industrial facility for offices, warehouses, garages, machine shops and recreational facilities that are not an integral part of the industrial plant, substation or control center (see Figure 1).

The new provision pertaining to the protection of conductors in supervised industrial installations was added to the 2011 NEC. In accordance with 240.91, conductors shall be protected in accordance with 240.91(A) or (B). Section 240.91(A) states that conductors shall be protected in accordance with 240.4. For example, what size XHHW copper conductors are required to supply a 1,200A, three-phase, 4-wire, 208V feeder under the following conditions? The feeder conductors will be paralleled, and there will be three sets of conductors in parallel. The load will be 1,120A, noncontinuous. The neutral conductor carries only the unbalanced load from the other conductors and, therefore, will not be required to be counted as a current-carrying conductor. There also will be an equipment grounding conductor in this raceway. The feeder will be installed indoors in a dry location. The voltage drop will not exceed the NEC recommendation. All of the terminations will be rated 75°C. The maximum ambient temperature will be 30°C. This feeder will be installed in a supervised industrial installation. Find the minimum size conductors in accordance with the provision in 240.91(A).

Because there are only three current-carrying conductors and the ambient temperature will not be above 30°C, it is not necessary to apply correction and adjustment factors. Although the XHHW conductors are rated 90°C, the allowable ampacity shall not exceed the 75°C column because of the termination provision in 110.14(C)(1)(b). Section 240.91(A) states that conductors shall be protected in accordance with 240.4. Section 240.4(C) states that, where the overcurrent device is rated over 800A, the ampacity of the conductors it protects shall be equal to or greater than the rating of the overcurrent device defined in 240.6. Since the overcurrent device will be 1,200A, the total combined ampacity of the conductors must be at least 1,200A. Because there will be three sets of conductors in parallel, the minimum ampacity per conductor must be at least 400A (1,200 ÷ 3 = 400). The ampacity of a 600 kcmil conductor, from the 75°C column of Table 310.15(B)(16), is 420A. Three sets of conductors in parallel have an ampacity of 1,260A (420 × 3 = 1,260). Since the ampacity of the paralleled conductors is at least the rating of the 1,200A overcurrent device at the feeder, this installation is permitted (see Figure 2).

Section 240.91(B) covers devices rated over 800A in supervised industrial installations. Where the overcurrent device is rated over 800A, the ampacity of the conductors it protects shall be equal to or greater than 95 percent of the rating of the overcurrent device specified in 240.6 in accordance with the two stipulations in 240.91(B)(1) and (2). The first stipulation states that the conductors must be protected within recognized time versus current limits for short-circuit currents. The second stipulation states that all equipment in which the conductors terminate must be listed and marked for the application. For example, what size XHHW copper conductors are required to supply a 1,200A, three-phase, 4-wire, 208V feeder under the following conditions? The feeder conductors will be paralleled, and there will be three sets of conductors in parallel. The load will be 1,120A, noncontinuous. The neutral conductor carries only the unbalanced load from the other conductors and will not be required to be counted as a current-carrying conductor. There also will be an equipment grounding conductor in this raceway. The feeder will be installed indoors in a dry location. The voltage drop will not exceed the NEC recommendation. All of the terminations will be rated 75°C. The maximum ambient temperature will be 30°C. This feeder will be installed in a supervised industrial installation. The conductors will be protected within recognized time versus current limits for short-circuit currents. All equipment in which the conductors terminate will be listed and marked for the application. Find the minimum size conductors in accordance with the provision in 240.91(B).

Since there are only three current-carrying conductors and the ambient temperature will not be above 30°C, it is not necessary to apply correction and adjustment factors. Although the XHHW conductors are rated 90°C, the allowable ampacity shall not exceed the 75°C column because of the termination provision in 110.14(C)(1)(b). Since this installation meets the two stipulations in 240.91(B)(1) and (2), the ampacity of the conductors can be equal to or greater than 95 percent of the overcurrent device rating. To find the 95 percent of the overcurrent device, multiply the 1,200A overcurrent device by 95 percent. The total combined conductor ampacity must be at least 1,140A (1,200 × 95% = 1,140). Since there will be three conductor sets in parallel, the minimum ampacity per conductor must be at least 380A (1,140 ÷ 3 = 380). The ampacity of a 500 kcmil conductor, from the 75°C column of Table 310.15(B)(16), is 380A. Three sets of conductors in parallel have an ampacity of 1,140A (380 × 3 = 1,140). Since the paralleled conductors ampacity is at least 95 percent of the 1,200A overcurrent device rating at the feeder, this installation is permitted (see Figure 3).